The present invention relates to a low cost, reliable system for generating uniform or homogenous raster patterns. The system is also useful for the generation of homogeneous pattern distributions of charged particles or rays of any type where magnets are used to obtain the required distribution.
The need to generate uniform or homogeneous raster patterns of electron beams directed at cryogenic targets such as liquid hydrogen, liquid deuterium, liquid helium, etc. in order to minimize localized heating and concomitant boiling is well known to those skilled in the art of particle accelerator operation. Similar requirements exist in such industrial applications as ion implantation and in medical therapy using charged particle beams. Common sinusoidal raster systems, for example, produce a beam pattern that because of the return points of the track produce higher beam densities along either the raster boundaries or at the corners of the raster pattern.
A variety of solutions have been proposed to solve this problem including the generation of circular, spiral and even triangular patterns. For example, Y. K. Batygin, et al in Nuclear Instruments and Methods in Physics Research A, 1995 pgs. 128-130 suggests a circular sweep of a particle beam over a target for making a uniform irradiation zone. The scan pattern is an untwisted spiral for a continuous beam or a family of concentric circles for a bunched beam. The deflection voltage provided by the RF cavities with orthogonal transverse fields shifted in time by xcfx80/2. The amplitude of the deflection voltage is a function of the time and the optimum beam scan frequency with respect to the RF frequency of the accelerator are also defined.
M. Fukuda et al in Nuclear Instruments and Methods on Physics Research A 396, (1997) Pgs. 45-49 describes a circular beam scanning method for uniform irradiation over a large area using a beam that follows a spiral trajectory on the target thereby guaranteeing continuous circular irradiation. Scanning speed and trajectory spacing in the radial direction are constant to make particle distribution uniform. The radial position of a beam spot and an angular frequency of the spiral scanning are expressed by an irrational function of time. In those cases where the radial pitch of the spiral trajectory is much smaller than the beam width, uniformity of the particle distribution depends on the ratio of the minimum radius of the trajectory to the beam width.
While these proposed solutions improve the uniformity of the raster or particle beam, they do not produce an optimized result and they rely on the use of relatively expensive and to some extent less than highly reliable components.
Such circular beam sweeping raster pattern generators that require a variation in angular frequency require much higher performance from the driving amplifiers than the system described herein.
Compared with 2-D triangular waveform raster systems that require Hbridges, the system described herein is much more reliable and requires significantly less maintenance.
According to the present invention a low cost, highly reliable power amplifier is used in combination with a resonant loop magnet driver system to drive a pair of X-Y magnets according to a phase shifting function to generate a pseudo spiral beam pattern that produces an extremely uniform or homogeneous particle beam pattern. Thus the system of the present invention embeds a homogeneous amplitude function proportional to (t)xc2xd in a function generator or uses a triangular current source to bias a diode generating an approximate (t)xc2xd voltage output signal. A main waveform generator produces a major driving sinusoidal waveform at a constant frequency, and is set to external amplitude modulation mode. The special waveform produced by the function generator or a biased diode circuit sent to the main waveform generator. The modulated waveform is then forwarded to the low-power power amplifier. Power is applied to a resonant loop circuit that steps up the power to the magnet directing the particle beam thereby producing a uniform density circular raster pattern.
In the case where two magnets, x and y, are used to direct the particle beam, the amplitude modulated output from the main generator is split into two channels. The first is fed to the power amplifier for one of the magnets and the second to the power amplifier for the second magnet each with the same amplitude but with a 90xc2x0 phase shift. Both of the power circuits includes a resonant loop.
The resultant raster pattern is a round shape with very uniform density when linear driving amplifiers are used to power the X and Y magnets. In those cases where magnets X and Y are powered by two resonance loops, an adjustment of the symmetry of generator A must be performed to obtain the final uniform raster pattern because of the narrow passing band of the resonance loops.